Former Extent of Park Glaciation
During the Pleistocene Period or Ice Age when most of Canada and a large portion of the United States were covered several times by an extensive ice sheet or continental glacier, all the valleys of Glacier National Park were filled with valley glaciers. These originated in the higher parts of the Lewis and Livingstone Ranges. On the east side of the Lewis Range they moved out onto the plains. From the Livingstone Range and the west side of the Lewis Range they moved into the wide Flathead Valley. During the maximum extent of these glaciers all of the area of the Park except the summits of the highest peaks and ridges were covered with ice.
The great Two Medicine Glacier, with its source in the head of the Two Medicine and tributary valleys, after reaching the plains spread out into a big lobe (piedmont glacier) eventually attaining a distance of about 40 miles from the eastern front of the mountains. The stream of ice emerging onto the plains from St. Mary Valley also extended many miles out from the mountain front. Several of these long valley glaciers extended far enough out onto the plains to meet the edge of the vast continental ice sheet moving westward from a center in the vicinity of Hudson Bay. In the major Park valleys these glaciers attained thicknesses of 3,000 or more feet. Although man probably never viewed this magnificent spectacle, the Park at that time must have been similar in aspect to some of the present day ice filled ranges along the Alaska-Yukon border.
No one knows exactly how many times glaciers moved down the Park valleys during the million or more years of the Pleistocene period, but geologists have found evidence for at least eight distinct advances. It is difficult to determine just when the first advance took place but it may have been very early in the period. Most of the advances, however, occurred during the past 70,000 years or so in what is known as the Wisconsin stage of the Ice Age. Large glaciers flowed down the Park valleys probably as late as 7,000 years ago. Between each of the major times of ice advance, the glaciers, responding to warmer or drier climate, shrank to small size and in some instances disappeared. These warmer intervals varied in length from 2,000 to tens of thousands of years.
Evidence of the several distinct glacial advances is yielded by the moraines, deposits of rock debris left by the ice. On the east side of the Park the lower courses of the major valleys and the adjoining ridges in the Park and on the adjacent plains are covered with moraines. The material in them ranges in size from clay to large boulders, and was deposited by glaciers after being transported down the valleys. The debris deposited by the latest ice advance is fresh in appearance and contains fragments of all Park rocks. Moraines of the earlier stages, because of much greater age, are more weathered. They contain many fragments of much weathered diorite, from the layer of rock that appears as a conspicuous black band on many of the mountains, and almost no fragments of limestone, so common in the newest moraines. The diorite is more resistant to weathering than the limestone which slowly dissolves in ground-water. The only localities where the oldest moraine occurs are the crests of the ridges which run eastward from the mountains out onto the plains. This material is especially abundant on St. Mary Ridge. On top of Two Medicine Ridge along and just above the highway, fragments of this material have been cemented together into a comparatively hard tillite. Lower down on the slopes the older moraine cannot be found as it is covered by that of the later glacial advances which were less extensive and did not override the ridge crests as did the earlier glaciers. The older debris is also found on top of Milk River and Boulder Ridges.
Following the last maximum advance of the Wisconsin glaciers they slowly shrank until about 6,000 years ago when all glacial ice probably disappeared from the mountains. After this there was a warm, dry period during which it is probable that no glaciers were present. Then about 4,000 years ago the present small glaciers were born. During the period of their existence they have fluctuated in size, probably attaining maximum dimensions around the middle of the last century. Since then they have been getting smaller.
PANORAMIC VIEW OF GRINNELL GLACIER AS IT APPEARED IN 1945. THE CREVASSES IN GLACIER MAY BE OVER 50 FEET DEEP (BEATTY PHOTO)
PANORAMIC VIEW OF SPERRY GLACIER AS IT APPEARED IN 1946. NOTE MELT-WATER LAKES TERMINATING AGAINST MORAINES AT EXTREME LEFT (DYSON PHOTO)